In conventional ships, a two-stroke low speed diesel engine that can output at low speeds and can be driven in a state of being directly connected to a propeller.
In recent years, natural gas with low NOx and SOx emissions has received attention as fuel for low speed diesel engines. By injecting high pressure natural gas as fuel into a combustion chamber of a low speed diesel engine and burning it, an output can be obtained with high heat efficiency.
In conventional ships, a two-stroke low speed diesel engine is used. The diesel engine can output at low speeds and is driven, directly connected to a propeller.
In recent years, natural gas with low NOx and SOx emissions has received attention as a fuel for low speed diesel engines. By injecting high pressure natural gas as a fuel into a combustion chamber of a low speed diesel engine and burning it, an output can be obtained with high heat efficiency.
For example, a reciprocating pump is driven by converting the rotational motion to the reciprocating motion using a crankshaft. In a case where a piston of the reciprocating pump is driven using the crankshaft, since the piston stroke is determined by the crankshaft, it is not possible to freely adjust the piston stroke. In addition, in a case where a plurality of reciprocating pumps is driven by an identical crankshaft, it is difficult to independently control the individual reciprocating pumps.
Meanwhile, Japanese Unexamined Patent Application Publication No. JP 2005-504927 (“JP 2005-504927”) describes a device that boosts liquid fuel using a reciprocating pump and supplies the boosted fuel to the engine. In the device in JP 2005-504927, a piston of the reciprocating pump is driven in the left-right direction, and a “linear hydraulic motor” (hydraulic cylinder unit) is used as a linear actuator that drives a piston. In JP 2005-504927, the moving direction of the piston of the reciprocating pump is switched by switching, with a direction switching valve, the direction of the hydraulic fluid supplied from the hydraulic pump to the hydraulic cylinder unit. With the use of the hydraulic cylinder unit, it is possible to drive the reciprocating pump at a lower speed than the case where the crankshaft is used. Moreover, this method has an advantage that the piston stroke can be controlled so as to allow the piston to move at a constant speed.
In the case of switching the moving direction of the piston of the reciprocating pump by switching the direction of the hydraulic fluid supplied to the hydraulic cylinder unit by a direction switching valve as described in JP 2005-504927, the force acting on the piston from the supplied hydraulic fluid is equal at ejection and at suction. On the other hand, the reciprocating pump needs a large force when ejecting fuel at a high pressure, while the load is small when sucking low pressure fuel. For this reason, in a case where the reciprocating pump is driven using the hydraulic cylinder unit, the piston is more likely to move at a higher speed at suction when the load of the piston is small, and thus, the pressure inside the cylinder of the reciprocating pump occasionally becomes lower than the vapor pressure of the fuel, increasing the possibility of occurrence of cavitation. The occurrence of cavitation leads to a problem of erosion occurring in the piston and cylinder of the reciprocating pump, causing shorter life.
Moreover, since the piston speed remains the same at ejection and at suction in a case where the reciprocating pump is driven using the crankshaft, it is difficult to change solely the speed at the time of suction, leading to the occurrence of cavitation at high rotation and high output.